In the past, an insulated conduit has been made by constructing a pipe section comprising a core pipe, a casing pipe positioned generally coaxially around or at least aligned with the axis of core pipe, and appropriate material between the core pipe and the casing pipe to thermally insulate the core pipe, and means to permit the connection of the pipe section to other similar sections. The insulating material, generally a foamable polyurethane material is placed in the generally annular space between the core pipe and the casing pipe. This placement is done by positioning the core pipe within the casing pipe by the use of end plugs and, if necessary, a rigid spacer in the annular space somewhere approximate the center of the pipe section. The core pipe and casing pipe were angled at approximately 15.degree. to the horizontal. A source of the foamable, curable urethane material was connected to a flexible hose and this hose is introduced into the space. The material, in an activated but un-foamed (liquid) condition was conducted down the hose to the annular space and flowed down the pipe to a point approximate the end of the casing remote from the upper end thereof. As the material reacts, it foams, expands and flows along the axis of the elongated space and up the distance defined by the casing and the core pipe, substantially filling the elongated space. The reacting urethane becomes relatively rigid and encapsulates inumerable small gas bubbles to insulate the core pipe.
Because the pipe section must be of substantial length (up to 10' to 20' or more in total longitudinal dimension) a number of problems arise from this method. Because of the distance that the material must be conducted, various parameters well known in the polymer foaming art must be adjusted to delay the reacting and foaming in order to permit the material to be deposited in the pipe before the material begins to react and foam. This delay prevents the clogging of the hose used to introduce this material and also, prevents the material from foaming up and "blocking" a portion of the space, thus entrapping a portion of air and creating a void in the insulating layer of the cured, foamed material.
While pouring and foaming the insulating material in the manner described above reduces condierably the void creating problems, it often increases material usage. It was found that when an adequate amount of material was placed towards the lower end of the sloping pipe, hydrostatic pressure and slowed reaction time prevented the material from fully expanding to its optimum density during the foaming operation. This hydrostatic pressure was a result of both the fluid "head" created over a portion of the material by the mass of unfoamed/foaming material, as well as the tendency of the material to become highly viscous during its expanding process, thus limiting the degree to which the underlying foaming material could expand due to sheer forces between the viscous material and the pipe wall surfaces.
Attempts to increase the rate of foaming reaction of the insulating material, and thus increase the tendency of the foam to fully expand, resulted in generating compressive forces and high exothermic reaction temperatures on the interior pipe which, if the interior pipe was made of a thermoplastic (specifically PVC), led to undue heating and subsequent collapse of the core pipe. Reducing the angle of the pipe relative to the horizontal would tend to reduce the "head" over the expanding material, and thus reduce the hydrostatic pressure and permit greater expansion. However, this would precipitate entrapping air pockets and result in the creation of cavities in the insulating material and concommitant reduction in the insulating value provided thereby.